1. Field of the Invention
The present invention is directed to a line transformer suitable for matching impedances in RF systems, such as in a nuclear magnetic resonance tomography apparatus.
2. Description of the Prior Art
Transformers suitable for operation with low frequency alternating currents generally contain a core of ferromagnetic material, with primary and secondary windings each consisting of a number of turns. The four pole formed in this manner is intended to modify current and voltage in a specified manner. Such transformers are suitable for impedance transformation as long as the length of the conductor forming one of the windings is short in comparison to the wavelength. As is known, radio antennas must also transmit electromagnetic energy, i.e., arbitrary signals, in an undistorted fashion at extremely high frequencies. In broadband transformers, therefore, the two windings are so tightly coupled that they form lines having a defined characteristic impedance and a negligible radiation loss. Virtually any rational voltage ratio can be achieved with line transformers (NTZ 1966, No. 9, pages 524-538).
It is also known that inductances can be made in so-called "pancake" design, also referred to as printed coils. Such inductances are formed by a conductor in the shape of a spiral which is arranged on the surface a flat piece of electrically insulating material. The opposite flat side of the insulator can be provided with a large-area metallization (1987 IEEE MTT-S Int. Microwave Symp. Dig., Vol. 1, pages 123-126).
Four port differential transformers are required in radio-frequency technology for realizing various decoupling and branching circuits, for example, directional couplers. These four ports, frequently referred to as hybrid sets in low-frequency technology, must be fashioned as line transformers to achieve a large bandwidth in order to reduce the transmission losses.